Q. I’ve written a number of technical user manuals. I would always write, “Perform step 1a, then do step 1b.” But then the Microsoft style guide stated that I should always write, “Perform step 1a, and then do step 1b.” I prefer the former and think it’s perfectly OK. What sayest thou?
A. Although CMOS 16 was silent on the issue, it is covered in the 17th and 18th editions in response to many reader queries like yours: “The adverb then is often seen between independent clauses as shorthand for and then, preceded by a comma.” Please see paragraph 6.61 in the 18th edition.